The invention concerns a detector of stains on a plate surface transparent to visible rays, in particular, a motor vehicle window.
The invention concerns, more specifically, a detector of stains, for example liquid, on the surface of a transparent plate that has two opposite sides notably parallel, and an internal face and an external face defining an interior volume.
In this type of detector, an emitter is arranged on the internal face of the plate and emits a modulated light signal in the direction of the interior of the plate such that the signal penetrates the plate. While the signal hits one of the faces of the plate, the signal is reflected towards the interior of the plate if this face is in contact with the air or altered, that is to say, refracted, focalized, absorbed, diffused, etc., in the case where the face is in contact with the stains to detect. Stains are defined here to be all deposits having an aqueous component, such as rain, snow, frost, mud, etc.
This type of device also comprises a receptor arranged in a target zone of a face of the plate such that the signal that moves around in the plate, when it hits the target zone, is reflected and is received by the receptor.
Such detection devices allow, for example, controlling the activation of motor vehicle wipers when drops of water appear on the windshield.
As a function of the material of the windshield and the length of the electromagnetic signal wave, the angle of incidence of the modulated light signal is chosen in such a manner that the ray is reflected in the direction of the internal face of the windshield if the external face is in contact with the air at the level of the impact zone of the signal and in such a manner that the ray is at least partially refracted towards the exterior if the water or stain covers the impact zone of the external face.
If the ray is reflected, it will be guided to follow its route to the interior of the windshield, in the thickness of the glass, in order to be reflected several times by the internal and external faces of the windshield. In a place of the internal face that is on the trajectory of the signal, one has places for optical coupling means, the refraction index of which is such that the signal is refracted across the internal face of the windshield in order to be transmitted to an adapted receptor.
In this way, if the receptor captures a signal higher than a determined threshold, no stain at the level of the impact zones of the signal against the external face of the windshield is detected. On the other hand, if the receptor no longer receives a signal (for example, due to absorption, diffusion, or refraction), where if it receives a signal at a level notably higher (for example from focalizing), the presence of stains, for example, drops of water, are present on the exterior face of the windshield.
One has however noted that in such a unit, the signal received by the receptor was slightly higher. In order to augment the level of detection, in particular the detection of fine, small drops in small numbers, the number of reflections of the signal on the external face of the plate was multiplied by detecting the modulated light after having been reflected several times and alternatively on the external face and on the internal face of the plate.
This method""s inconveniences are that it is particularly sensitive to the deposits of condensation on the internal face of the plate, imposing a relatively precise positioning of the emitter and receptor parts, and that it is particularly sensitive to the variations and imperfections of the thickness of the plate.
The invention aims to eliminate these inconveniences by proposing a detector that is at the same time compact and likely to supply an adapted number of reflections on the face to survey, by exploiting the principle of total reflection.
More precisely, the invention concerns a detector of stains on the surface of a transparent plate that has two notably parallel, opposite sides, an internal face and an external face. Such a detector, arranged on the internal face of the plate, comprises means emitting a modulated light signal in the direction of the interior of the plate, and means receiving this signal after having been reflected on the external face of the plate, and an optical unit made with material having an index substantially higher than one and transparent to the radiance of the modulated light signal. The optical unit is placed on the internal face of the plate and is made up of at least two faces deflecting the light signal, in order to form a trajectory between the emission means and the receiving means, and from the bottom interface with the internal face of the plate across the means of optical coupling. Also planned: deviation means of the signal from the emission means, so that the signal reflects a first time on the external face of the plate according to a direction included in an initial incidence plane, and deviation means of the signal towards the receiving means so that the faces of deflection thus orient the signal successively towards the external face in at least one incidence plane notably not parallel in the initial incidence plane so that the trajectory of the light signal between the emission means and receiving means produce at least two reflections on the external face of the plate across the optical coupling means without reflection on the internal face of this plate.
Thus, the trajectory of the signal, between the emission and reflection means, is notably confined in the optical unit and in the volume of the plate situated opposite from the unit.
According to the specific modes of production of the invention:
the optical unit is, on the whole, outlined according to a parallelepiped rectangle having mainly a front face, an upper face, a rear face, two lateral faces, and a bottom face that interfaces with the plate;
the deflection faces are made up of a rear face, the upper face and the front face, or by the front face and the rear face, these two faces being inclined in order to make a deflection signal towards the plate;
the light signal penetrates the optical unit after deviation via an input face situated in the front face of the optical unit, and leaves from the front deviation optical unit on the receiving means via an output face situated in the front face while the number of reflection on the external face of the plate is even, and in the rear face when the number of reflections is odd;
the input and output faces of the unit form lenses, which can be coupled to an optical deflection in order to make up the deviation means;
at least one of the lenses is a convex lens comprising at least a non-spherical or spherical surface, where there is a Fresnel lens, the lenses can make up an integral part of the optical unit;
the optical coupling means are formed via a flexible layer of transparent material in the light signal, for example, silicon, the layer being compressed between the bottom face of the optical unit and the internal plate face;
an interferential filter is planned between the output face and the receiving means attached on the spectral band of the signal in order to eliminate light interference;
the optical unit and the optical coupling means are mass-colored to eliminate light interference;
a heating element is placed near or against one of the faces of the optical unit in order to eliminate condensation; thanks to the compactness of the detector the condensation is eliminated in an efficient and rapid manner;
the emitting means can be made up of an electro luminescent diode or by a photodiode, and the receiving means by a silicon photo detector cell, the emitting diode and the receiving cell can be placed near each other;
the emitting and receiving means are connected to a treatment unit formed on a printed circuit board in order to produce a modulation in amplitude for the signal of the emitting diode and a synchronous detection of the signal received by the photo detector cell;
the deflection faces are subjected to a treatment reflecting the light signal, for example, an aluminizing, where they are recovered with a layer of material reflecting the light signal;
the faces of the optical unit, other than the deflection faces and the interface, are covered with a material that absorbs the light signal.